1. Field of the Invention
The present invention relates to an illumination optical system and an exposure apparatus.
2. Description of the Related Art
As an exposure apparatus that uses a light source having a smaller wavelength for the improved resolution, there is proposed an (EUV) exposure apparatus that uses the extreme ultraviolet (EUV) light having a wavelength between 10 and 15 nm. Japanese Patent Laid-Open No. (“JP”) 2005-141158 (corresponding to U.S. Patent Application Publication No. (“US”) 2005/0105290) proposes an illumination optical system for the EUV exposure apparatus, which arranges an aperture stop having a semicircular aperture on a reflection integrator having a plurality of cylindrical reflection surfaces that have parallel meridional line directions.
Other prior art include JP 2008-098527.
In order for the projection optical system to realize high-quality imaging in the exposure apparatus, it is necessary to reduce the distortion of the effective light source of the illumination optical system viewed from each position on a surface to be illuminated. The illumination optical system disclosed in JP 2005-141158 sufficiently maintains the distortion of the effective light source smaller than prior art of those days. However, according to the research by this inventor, it is found that the non-negligible distortion still remains in the effective light source in the illumination optical system of JP 2005-141158, because the aperture stop shields part of the parallel light before the light forms a secondary light source. In addition, the illumination optical systems disclosed in JPs. 2005-141158 and 2008-098527 deteriorate the light use efficiency because it realizes a strong modified illumination, such as a dipole illumination, through light-shielding by the stop.